The present invention relates to a large-aperture lens with aspherical surfaces which is suitable for use in the readout of optical information, in a transmission system of optical communication, or the like.
In the prior art, a large-aperture lens with aspherical surfaces has been used in laser pickups used in compact disks (CD), laser disks (LD), etc. In the recent years, it has come to be widely used in magnetic optical memory of computers and fine optical system of optical communication transmission systems.
A large-aperture lens with aspherical surfaces has a wide field of applications. Generally, its numerical aperture (NA) is 0.45 to 0.55 and its focal length is approximately from 3.5 to 10.0.
For its materials, plastic is often used become it is inexpensive, has excellent workability, and is light in weight. A plastic lens in which both surfaces are formed as aspherical surfaces is widely used, for example, in CD players.
However, plastic has problems with respect to heat resistance, moisture absorption, and birefringence (homogeneity). Because of its insufficient heat resistance, there is a possibility that the properties of plastic may be changed by the high temperature treatment used in joining it with an optical lens mount. When a sealing glass is used at the time of fixing a lens, the outer circumference of the lens is exposed to temperatures of 400.degree. C. to 500.degree. C. during gold plating. Therefore, plastic lenses cannot be used in such conditions.
As a lens for use in connectors for optical fibers, a gradient index lens has traditionally been available. Such a lens has gradient index characteristics because it has been manufactured by an ion exchange method and its quality varies greatly from lot to lot. Further, these exert an adverse influence on spherical aberration, etc. It is difficult to deal with a high numerical aperture, and therefore it cannot be used stably in high-precision optical systems in which semiconductor laser light sources are used.
Furthermore, U.S. Pat. No. 4,449,792 describes a large-aperture single lens with aspherical surfaces and U.S. Pat. No., 4,571,034 describes a lens system for optical recording type disks. In either of these lenses, if one desires to obtain a lens having a shorter focal length of 1 mm or so, as is required for optical communication connector lens, the vertex radius and wave front aberration obtained do not show desired values.